The present invention relates an apparatus and method for removing particles from gas, exhausted from an internal combustion engine, by a plurality of filters.
Particles (or exhaust particles), containing carbon as a main component, are contained in an gas exhausted from an internal combustion engine of an automobile, particularly from a diesel engine.
Therefore, an exhaust particulate remover apparatus having a filter made of ceramics is provided in an exhaust path of a diesel engine and particulate in an exhaust gas is collected by the filter, in order to remove the particulate.
As particulate thus collected is accumulated and increases, ventilation of the filter is deteriorated gradually and, finally, the filter must be refreshed. Besides, it is necessary to continue cleaning the exhaust gas while reclaiming the filter.
In order to simultaneously achieve both the cleaning and reclaiming, a proposal has been made as to a structure as follows (in Japanese Patent Application KOKAI Publication No. 3-134215). Two sets (or a plurality) of filters are provided in parallel in an exhaust path of a diesel engine. As shown in FIG. 8, collection of particulate is carried out simultaneously by two filters (designated at No. 1 and No. 2). Based on a determination as to collection amounts, only one (No. 1) of the filters is refreshed when a refresh timing comes. While reclaiming the filter, the gas, exhausted gas is made to flow through the other filter (No. 2) so that the exhaust gas is kept cleaned and exhausted. At the timing when reclaiming of the filter (No. 1) is completed, the mode of collecting the exhaust gas is switched to simultaneous collection using two filters (No. 1 and No. 2).
However, in the above kind of exhaust particulate remover apparatus as described above, a large thermal load may be applied to a filter when reclaiming a filter.
This relates to cinders of particulate caused by the reclaiming process.
Specifically, reclaiming of each filter is carried out by burning accumulated particulate on the filter with the use of a heating source such as an electric heater or the like. Since the reclaiming is carried out within a certain predetermined time period, cinders of particulate tend to be easily caused on the filter. The amount of cinders varies particularly depending on how particulate is burnt (e.g., the burning temperature or the like) and a difference in the amount of cinders represents a difference in accumulation amount of particulate collected between the filters.
The difference in accumulation amount of particulate may be considered to be eliminated when particulate is collected by simultaneously using two sets of filters. In practice, however, a refresh timing comes before the difference is eliminated, and therefore, the difference in accumulation amount cannot be eliminated. In addition, the collection amount, from which a refresh timing is determined, is a total accumulation amount of particulate collected by two sets of filters, but the accumulation amount of each filter cannot be acquired.
Therefore, as shown in FIG. 9, there is a case where particulate accumulated in the filter No. 2 reaches an accumulation amount at which the particulate burns at a burning temperature higher than the tolerable temperature of the filter, for example, if the filter No. 1 is refreshed prior to the filter No. 2 among two sets of filters where the accumulation amount of the filter No. 1 is smaller than an average value of the accumulation amounts of the filters No. 1 and No. 2 and the accumulation amount of the filter No. 2 is greater than the average value.
This means that the inner temperature of the filter exceeds a tolerable temperature (or a tolerable value) when reclaiming the filter No. 2, and a large thermal load is applied thereby deteriorating the durability of the filter.
Hence, a proposal has been made as to the order in which the filters No. 1 and No. 2 are subjected to reclaiming is changed periodically, e.g., every time when the two sets of filters are refreshed (in Japanese Patent Application KOKAI Publication No. 6-307225).
However, if the refresh order is changed every time when the two sets of filters are refreshed, the collection amounts of the filters No. 1 and No. 2 change alternately as shown in FIG. 10. Therefore, as can be seen from reclaiming for first and second times, the particulate amount accumulated on the filter change greatly when reclaiming the filters.
Thus, when reclaiming any of the filters No. 1 and No. 2, the internal temperature of the filter often exceeds a tolerable temperature (or tolerable value), and as a result, the durability of the filters No. 1 and No. 2 is deteriorated. Needless to say, the durability of the filters is deteriorated even if the refresh order is changed periodically.
Hence, there has been a demand for an apparatus which is capable of reclaiming filters while improving the durability of filters.